The present invention generally relates to electric motors/generators and electrically driven compressors and, more particularly, to a cooling jacket of a dry-liquid cooled electric motor/generator and a method for dry liquid cooling an electric motor/generator of an electrically driven compressor.
Electric motors or generators typically generate a substantial amount of heat during operation, especially if operated at high speeds. Consequently, an electric motor or generator needs to be cooled in order to avoid damage and to ensure smooth and efficient operation of the motor or generator. Since the heat transfer coefficient for cooling using a liquid is generally much higher than the heat transfer coefficient for air, the stator of an electric motor or generator is often cooled with a liquid coolant.
During wet liquid cooling, the stator iron stack and the stator winding end turns are typically completely immersed in a cooling liquid, such as oil. Heat is extracted from the stator by conducting the heat from the stator core and winding to the cooling liquid. Wet liquid cooling of the stator requires sealing the rotor from the space surrounding the stator and is limited to the use of nonconductive liquids, since the stator winding end turns typically are immersed in the cooling liquid as well.
Dry liquid cooling of the stator is often used as an alternative to wet liquid cooling and utilizes in many cases a cooling jacket that surrounds the iron stack and winding of the stator, for example, U.S. Pat. Nos. 5,220,233, 5,923,108, 6,617,715, and 6,909,210. A cooling liquid, which may be a conductive liquid such as water or a water-based cooling liquid, is typically circulated through channels within the cooling jacket and heat is transferred from the stator through direct contact of the stator with the cooling jacket.
Even though the application of cooling jackets for dry liquid cooling is well known in the art, prior art cooling jackets often have high manufacturing costs, require extensive and complex sealing arrangements, often are not leak proof, and may not distribute the cooling liquid evenly—causing hot spots on the stator core. U.S. Pat. No. 6,900,561, for example, discloses a cooling jacket where the cooling liquid enters the cooling jacket axially in an inclined plane and exits the cooling jacket radially from an inclined plane, which may cause pressure losses in the liquid cooling loop. In other prior art cooling jackets, for example, U.S. Pat. Nos. 5,220,233 and 3,567,975, the cooling liquid travels in helical channels and enters/exits these channels radially, which may lead to even higher pressure losses and uneven distribution of the cooling liquid.
Many prior art cooling jackets, for example, U.S. Pat. Nos. 6,900,561 and 6,617,715, utilize “o”-rings to seal the inner and outer pieces of the cooling jackets. Such “o”-rings may not be leak proof and pressure tight and may show wear over long periods of operation, which may lead to high maintenance or repair costs.
Other prior art cooling jackets, for example, U.S. Pat. No. 5,923,108 may be manufactured form cast iron and, therefore, may be too heavy for aerospace applications. Furthermore, cooling jackets manufactured from iron-based materials may be prone to corrosion. Corrosion products that may build-up within the cooling jacket over time may cause degradation of the heat transfer capability of the cooling jacket.
As can be seen, there is a need for a cooling jacket that is leak proof and pressure tight, that minimizes pressure losses in the liquid cooling loop, and that has a reduced susceptibility to corrosion. Furthermore, there is a need for a cooling jacket that may be manufactured at a lower cost and that may be easier to assemble and to integrate into electrically driven machines than prior art cooling jackets. Still further, there is a need for a method for dry liquid cooling an electric motor or generator, which has a higher cooling efficiency than prior art dry liquid cooling methods and is also applicable in the aerospace industry.